Molecular Shape Prediction and the Lone-Pair Electrons on the Central Atom Saleh M. Al-Mousawi Kuwait University. P.O. Box 5969-Safat, 13060-Kuwalt The remarkably successful valence shell electron-pair repulsion (VSEPR) theory is normally used by students to predict molecular geometry ( I , 2). This is possible if the ~ e w i stucture s of a molecule is correctly written, as shown in general chemistry textbooks and literature (3-7). Nevertheless. students sometimes find i t difficult to decide how manv lond-pair electrons are on the central atom. The techniq& nronosed here makes this strikinelv simnle and enables ihapes to be predicted without diff<y. it involves calculatine the total number of valence electrons in the molecule or ion, 'I.,and dividing by 8 LO give a whole number, n. Thus, the basic molecular shane can be obtained as shown in Table 1. However, TI8 does not always give a whole number. In many cases a remainder is obtained. A statement that is based on this can be expressed as follows: The remainder obtained by dividing the total number of electrons in a molecule or ion by 8 is the number of unbonded electrons on the central atom. Thereby, the molecular geometry of AX,E- molecules can be obtained as shown in Table 2. ~h$'$uccessof this technique stems from the fact that TI8 = n b. where n is considered the number of atoms bonded to the cdntralatom and b is the remainder obtained from the division; b/2 = m, the number of pairs of lone-pair electrons. The rule works trivially for AX, molecules and can also be used to predict the number of lone-pair electrons on the central atom of AX.E,-type molecules.
+
Procedures for Shape Predlctlon 1. Calculate T , the total number of valence electrons in the molecule or ion. 2. Divide T by 8. 3. If the result of the division is only a whole number n, then the molecular geometry is easily predicted as listed in Table 1. Otherwise the remainder is the number of unbonded electrons on the central atom of AX,E, molecule, Table 2.
Exam ple
T Total Number of Valence Elecmns
BeCI2
16 18 24 32 40 48 56
Cog
SO, CCI. PF5
SFa IF7
TI8 = n
Oroup
Molecule Shape
2 2
AX2 AX2 AXs AX4 AXr AXa AX,
Linear Linear Planar triangular Tetrahedral Trigonal bipyramldal Octahedral Pentagonal bipyramldal
3 4
5 6 7
Table 2. Predlctlon of Molecular Shape When Molecules Have Unbanded Valence Electrons T, Total Number
Exam ple
of Valence Electrons
SO2
TI8 = n b n b
+
2
m Electron
Pairs
Group
1 1
AX&
TeCl.
18 26 20 28
CIF3 XBFZ IFs
28 22 42
3
4
2
2 5
8 2
3
XeF,
36
4
4
2
Sbtlrg-
50
6
2
1
%I, CIO;
Molecule Shape Angular
4
2 2 4 2
2 1
AX.E Pyramidal A X & Angular AX,E .Distoned AX&
3
2
tetrahedral T-Shaped AX2E3 Llnera AXrE Square pyramidal AXCI Square planar AXsE Irregular octahedral
1
.
culea is THIS= n + b, and TH = T + W, J is the number of hydrogen atoms bonded to the central atom in the molecule, for
Examples
(a) Sulfur triaxide, SO1 T = 24;
Table 1. Predlctlon of Molecular Shape When Molecules Have Even Octets of Valence Electron,
n = 2418 = 3;
SO3is planar triangular (h) Sulfur dioxide, SOz T = 18; n = 1818 = 2 2; SO, is angular
+
rn = 0;
AXa
---r--
(a) Methane, CHn T ~ = 8 + 6 X 4 = 3 2 ; T ~ / 8 = 4 ; A&.
b = 2;
rn = 1;
AXzE
The success of this technique can be extended to: t1J Molecules with an odd numher of electrons, e.g., CIO. and NO>. (2) Moleruler in which the central atom has less than eight electrans, e.g., BeC12 and AlC18. Molecules in which the central atom has more than eight electrons, e.g., SF6 and PCls. (4) Diatomic molecules like Nz,02, and Br2. These molecules are linear. and the technioue enables the number of unhonded electrons he determined. (5) Molecules in which a hydrogen atom is directly bonded to the central atom,e.g., CH, and HzO.A modified rule is necessary for such molecules because hydrogen atom does not obey the octet rule. Thus the modified equation associated with such mole-
CHdis tetrahedral (b) Water, Hz0
TH=8+6X2=20 T~/8=2+4; b=4;rn=2
AXzEz
HzO is angular. Acknowledgment Thanks are due to A. A. M. Ali for discussion. Literature Clted 1. Gillespie. R. G. Angeu. Chem. Internal. Edit. 1967.6,819. 2. Gillespie. R. G. J. Chrm.Edur. 1970,47,18. 3. Lever, A. B. P. J Chem. Educ. L972,49,819. 4. C1srk.T.J. J. Chem.Edur. 1984.61, IW. 5. Zandlor. M. E.:Talaty,E. R. J. Chcm.Educ. 1984,61. 124. 6. Spaiding,T. R.Edue. Chem. 1987,24,21. 7. Snadden. R. B. Educ. Chem. 1987,24.61.
Volume 67
Number 10 October 1990
881
